This invention is generally related to semiconductor image sensor dies and related wafer/die fabrication techniques, and more particularly to image sensor dies having color filter layers and microlens structures.
Image sensor dies that can electronically capture a scene in color are at the hearts of a wide range of consumer products such as digital still cameras and video cameras. FIG. 1 shows a cross-section of a conventional color image sensor die. The die is one of many identical dies that are manufactured simultaneously in a semiconductor wafer. The dies are formed layer by layer, normally beginning with an array of photodetecting sites 104 (photosites). Other integrated circuit elements, such as transistors, which cooperate with the photodetecting sites 104 to provide an image signal are not shown. The photosites 104 and the other circuit elements are interconnected using one or more metal layers 112. Only the top most such layer is shown in FIG. 1. The top most metal layer 112 typically forms a grid above the array of photosites to roughly define a well above each site 104. A color filter material is then deposited into the wells across the entire wafer, usually above a passivation layer 116. This material is repeatedly deposited and patterned so that each photosite 104 will have one of several different color filters 120 directly above it. Each photosite will thus respond only to light of a specific color which can pass through its corresponding filter. To improve the efficiency with which a photosite 104 responds to incident light, a microlens structure 128 that focuses the incident light is formed over each photosite. This combination of photosite 104, color filter 120, and microlens structure 128 will be referred to here as a color photocell.
One of the problems with a sensor die built using the above described color photocell is the presence of small differences in the image signals obtained from theoretically identical photocells that are subjected to the same incident light. These differences are sometimes referred to as photo response non uniformity (PRNU) noise and can significantly, and adversely, affect the accuracy or sharpness of the electronic image captured by the sensor die. The non-uniform thickness of the microlens structure 128 and the underlying color filter 120 across many thousands of photocells in the sensor array may be the cause of such a noise problem.
A limited solution that helps alleviate the PRNU noise is to place a planarization layer 124 between the color filters 120 and the microlens structures 128, as shown in FIG. 1. This makes the total thickness of the optical structure that includes the microlens, the planarization layer, and the color filter more controllable and hence more uniform across the sensor die. However, such a solution requires the additional process step of depositing and finishing the planarization layer 124. Such an additional process step contributes significantly to the overall cost of the sensor die.
An embodiment of the invention is a semiconductor die having a number of photodetecting sites. A microlens structure is positioned above each one of the photodetecting sites with a color filter above the microlens structure.
Other features and advantages of the invention will be apparent from the accompanying drawings and from the detailed description that follows below.